You will get many opinions on this subject, but as a person who takes photos of plant materials for work, and has spent money on the topic, I will express an opinion...
Given a choice, I do not like either the 24EX or the 14EX for this work. One of the first criteria is that EFFECTIVE size of the flash should be larger than the subject. The 24EX is excellent for hand-held pursuing of insects in the field, but not always for larger plant materials in situ. While 24EX is decent (I own one), for flowers, a person needs to also carry a flash to illuminate the background because flash power fall-off is so rapid a closeup distance (unless you are aiming for the black background look). Background can be illuminated with any Canon EX flash used as a wireless slave. You set Custom Function 5 on the 24 EX, let the wireless slave be in Channel C, non-metered background. But, by the time you have the 24EX and a slave, you're into some serious coin.
The second criteria is we frequently must REDUCE the contrast (reduce "subject brightness range" between highlights and shadows) on a flower subject, and directly behind the subject. This means a 12" or 20" collapse-able 1-stop translucent diffuser for $18 frequently has more utility than a $600 flash in yielding a good photo! Really good for slight under exposure which is required to yield full saturation of colors, just like shooting slide transparencies, and also for white flower petels. There is a reason why flower photographers all carry them: http://www.naturephotographers.net/np101/je0804-1.html
I would start with a pair of collabse-able diffusers, one translucent and one white/silver before buying any flash. Sometimes that's all that's needed to obtain an accurate documentary photo. I like the small Lastolite tri-grip best for field work, but any cheap one will do as long as you can keep it where you want. Wind is a big enemy.
For flash? I know it's a lot to carry, but my favorite rig is Quantum QFlash, with a portable softbox, mounted on a stand next to the camera, and connected via a TTL cord. Reflector provides fill. My 24EX sits around unused, unless handheld bug hunting. The QFlash does plant photos in the field (as long as walking distance to a car).
I know Quantum rig can be well over a grand, but I was in Allen's camera in Levittown, PA (where I get used gear), and he had just taken in a used QFlash T-5d-R, battery, hotshoe Canon TTL adaptor, and a portable softbox. Wanted about $750 for about $1,350 in gear. Allen's is a good business place for this used stuff.
Just one person's opinion. Jack